1. Introduction
This invention relates to a process and apparatus for generating permanganate through electrolytic oxidation. In particular, the invention relates to electrolytic regeneration of permanganate etchant baths.
2. Description of the prior art
Electroless metal plating of plastic substrates is employed to produce a variety of items such as printed electronic circuit boards and electromagnetic interference shielding. Prior to metal deposition, the plastic substrate is etched by an oxidant to enhance adhesion of the metal. This is often an essential step to a successful plating sequence. For example, conductive through-holes of printed circuit boards have posed persistent problems to metal plating which have been addressed through an oxidative etching process, as described in U.S. Pat. No. 4,515,829, incorporated herein by reference.
A variety of oxidant etching agents have been employed. Chromium compounds and concentrated sulfuric acid are less favored due to problems associated with application, safety and disposal. Much more convenient are the widely used permanganate solutions, particularly alkaline permanganate solutions.
The operating life of a permanganate etchant bath can be relatively limited as permanganate ions are reduced during the etching process to manganese species of lower oxidative states, such as manages dioxide and manganate. This reduction results directly from the etching process as well as from the etchant bath conditions; for instance, the alkaline bath promotes permanganate disproportionation to yield manganate. As it is permanganate rather than the lower oxidative state manganese species which exhibit polymer etching properties, to maintain etchant activity the bath must either be regularly replaced with fresh permangante solution or supplemented with additional permanganate ions. Preferably, permanganate concentration is maintained by oxidation of reduced manganese species present in the bath as addition of new permanganate to an existing bath or bath replacement are both expensive and burdensome.
A convenient means of permanganate regeneration is oxidative electrolysis, as generally described in U.S. Pat. No. 4,859,300, incorporated herein by reference. The efficiency of such electrolysis has been limited by reduction reactions occurring at the cathode, specifically the reduction of permanganate and lower oxidative state manganese compounds. Reduction yielding manganese dioxide particularly limits cell efficiency. Manganese dioxide is extremely insoluble in typical etching solutions and thus, once formed, cannot be oxidized at the anode to permanganate.
This problem is commonly addressed by use of a high anode surface area to cathode surface area ratio. However, a high electrode surface area ratio only limits, and does not eliminate, cathode reduction reactions. Furthermore, use of a high electrode surface area ratio can be burdensome where a relatively compact cell is required, for example if permanganate regeneration is performed in situ by placement of the cell within the etchant bath vessel.
A separated-type cell also has been employed to limit undesirable cathode reduction reactions. In this type of cell the cathode is separated from the anode by a porous membrane which restricts migration of permanganate ions to the cathode. However this system tends to be inconvenient. Manganese dioxide is produced through permanganate decay in the solution and as noted is virtually insoluble in the etchant solution. The insoluble manganese dioxide collects throughout the separated cell and particularly on the porous membrane separating the anode and cathode. The membrane consequently becomes blocked, preventing electricity flow through the cell. Regular cell disassembly and cleaning are required to maintain cell efficiency.